CHEMISTRY THE CENTRAL SCIENCE

21 NUCLEAR CHEMISTRY

21.7 NUCLEAR POWER: FISSION

Commercial nuclear power plants and most forms of nuclear weaponry depend on nuclear fission for their operation. The first nuclear fission reaction to be discovered was that of uranium-235. This nucleus, as well as those of uranium-233 and plutonium-239, undergoes fission when struck by a slow-moving neutron (FIGURE 21.13).*

FIGURE 21.13 Uranium-235 fission. This is just one of many fission patterns. In this reaction, 3.5 × 10–11 J of energy is produced per 235U nucleus.

A heavy nucleus can split in many ways. Two ways that the uranium-235 nucleus splits, for instance, are

More than 200 isotopes of 35 elements have been found among the fission products of uranium-235. Most of them are radioactive.

Slow-moving neutrons are required in fission because the process involves initial absorption of the neutron by the nucleus. The resulting more massive nucleus is often unstable and spontaneously undergoes fission. Fast neutrons tend to bounce off the nucleus, and little fission occurs.

Note that the coefficients of the product neutrons in Equations 21.24 and 21.25 are 2 and 3. On average, 2.4 neutrons are produced by every fission of a uranium-235 nucleus. If one fission produces two neutrons, the two neutrons can cause two additional fissions, each producing two neutrons. The four neutrons thereby released can produce four fissions, and so forth, as shown in FIGURE 21.14. The number of fissions and the energy released quickly escalate, and if the process is unchecked, the result is a violent explosion. Reactions that multiply in this fashion are called chain reactions.

FIGURE 21.14 Fission chain reaction.

For a fission chain reaction to occur, the sample of fissionable material must have a certain minimum mass. Otherwise, neutrons escape from the sample before they have the opportunity to strike other nuclei and cause additional fission. The amount of fissionable material large enough to maintain a chain reaction with a constant rate of fission is called the critical mass. When a critical mass of material is present, one neutron on average from each fission is subsequently effective in producing another fission and the fission continues at a constant, controllable rate. The critical mass of uranium-235 is about 50 kg for a bare sphere of the metal.*

If more than a critical mass of fissionable material is present, very few neutrons escape. The chain reaction thus multiplies the number of fissions, which can lead to a nuclear explosion. A mass in excess of a critical mass is referred to as a supercritical mass. The effect of mass on a fission reaction is illustrated in FIGURE 21.15.

FIGURE 21.15 Subcritical, critical, and supercritical fission.

FIGURE 21.16 shows a schematic diagram of the first atomic bomb used in warfare, the bomb dropped on Hiroshima, Japan, on August 6, 1945. To trigger a fission reaction, two subcritical masses of uranium-235 are slammed together using chemical explosives. The combined masses of the uranium form a supercritical mass, which leads to a rapid, uncontrolled chain reaction and, ultimately, a nuclear explosion. The energy released by the bomb dropped on Hiroshima was equivalent to that of 20,000 tons of TNT (it therefore is called a 20-kiloton bomb). Unfortunately, the basic design of a fission-based atomic bomb is quite simple, and the fissionable materials are potentially available to any nation with a nuclear reactor. The combination of design simplicity and materials availability has resulted in the proliferation of atomic weapons.

FIGURE 21.16 An atomic bomb design. A conventional explosive is used to bring two subcritical masses together to form a supercritical mass.

A CLOSER LOOKTHE DAWNING OF THE NUCLEAR AGE

Uranium-235 fission was first achieved during the late 1930s by Enrico Fermi and coworkers in Rome and shortly thereafter by Otto Hahn and coworkers in Berlin. Both groups were trying to produce transuranium elements. In 1938, Hahn identified barium among his reaction products. He was puzzled by this observation and questioned the identification because the presence of barium was so unexpected. He sent a letter describing his experiments to Lise Meitner, a former coworker who had been forced to leave Germany because of the anti-Semitism of the Third Reich and had settled in Sweden. She surmised that Hahn's experiment indicated a nuclear process was occurring in which the uranium-235 split. She called this process nuclear fission.

Meitner passed word of this discovery to her nephew, Otto Frisch, a physicist working at Niels Bohr's institute in Copenhagen. Frisch repeated the experiment, verifying Hahn's observations, and found that tremendous energies were involved. In January 1939, Meitner and Frisch published a short article describing the reaction. In March 1939, Leo Szilard and Walter Zinn at Columbia University discovered that more neutrons are produced than are used in each fission. As we have seen, this result allows a chain reaction to occur.

News of these discoveries and an awareness of their potential use in explosive devices spread rapidly within the scientific community. Several scientists finally persuaded Albert Einstein, the most famous physicist of the time, to write a letter to President Franklin D. Roosevelt explaining the implications of these discoveries. Einstein's letter, written in August 1939, outlined the possible military applications of nuclear fission and emphasized the danger that weapons based on fission would pose if they were developed by the Nazis. Roosevelt judged it imperative that the United States investigate the possibility of such weapons. Late in 1941, the decision was made to build a bomb based on the fission reaction. An enormous research project, known as the Manhattan Project, began.

On December 2, 1942, the first artificial self-sustaining nuclear fission chain reaction was achieved in an abandoned squash court at the University of Chicago. This accomplishment led to the development of the first atomic bomb, at Los Alamos National Laboratory in New Mexico in July 1945 (FIGURE 21.17). In August 1945 the United States dropped atomic bombs on two Japanese cities, Hiroshima and Nagasaki. The nuclear age had arrived.

FIGURE 21.17 The Trinity test for the atom bomb developed during World War II. The first human-made nuclear explosion took place on July 16, 1945, on the Alamogordo test range in New Mexico.

Nuclear Reactors

Nuclear power plants use nuclear fission to generate energy. The core of a typical nuclear reactor consists of four principal components: fuel elements, control rods, a moderator, and a primary coolant (FIGURE 21.18). The fuel is a fissionable substance, such as uranium-235. The natural isotopic abundance of uranium-235 is only 0.7%, too low to sustain a chain reaction in most reactors. Therefore, the 235U content of the fuel must be enriched to 3–5% for use in a reactor. The fuel elements contain enriched uranium in the form of UO2 pellets encased in zirconium or stainless steel tubes.

FIGURE 21.18 Diagram of a pressurized water reactor core.

The control rods are composed of materials that absorb neutrons, such as cadmium or boron. These rods regulate the flux of neutrons to keep the reaction chain self-sustaining and also prevent the reactor core from overheating.*

The probability that a neutron will trigger fission of a 235U nucleus depends on the speed of the neutron. The neutrons produced by fission have high speeds (typically in excess of 10,000 km/s). The function of the moderator is to slow down the neutrons (to speeds of a few kilometers per second) so that they can be captured more readily by the fissionable nuclei. The moderator is typically either water or graphite.

GO FIGURE

Why are nuclear power plants usually located near a large body of water?

FIGURE 21.19 Basic design of a pressurized water reactor nuclear power plant.

The primary coolant is a substance that transports the heat generated by the nuclear chain reaction away from the reactor core. In a pressurized water reactor, which is the most common commercial reactor design, water acts as both the moderator and the primary coolant.

The design of a nuclear power plant is basically the same as that of a power plant that burns fossil fuel (except that the burner is replaced by a reactor core). The nuclear power plant design shown in FIGURE 21.19, a pressurized water reactor, is currently the most popular. The primary coolant passes through the core in a closed system, which lessens the chance that radioactive products could escape the core. As an added safety precaution, the reactor is surrounded by a reinforced concrete containment shell to shield personnel and nearby residents from radiation and to protect the reactor from external forces. After passing through the reactor core, the very hot primary coolant passes through a heat exchanger where much of its heat is transferred to a secondary coolant, converting the latter to high-pressure steam that is used to drive a turbine. The secondary coolant is then condensed by transferring heat to an external source of water, such as a river or lake.

Approximately two-thirds of all commercial reactors are pressurized water reactors, but there are several variations on this basic design, each with advantages and disadvantages. A boiling water reactor generates steam by boiling the primary coolant; thus, no secondary coolant is needed. Pressurized water reactors and boiling water reactors are collectively referred to as light water reactors because they use H2O as moderator and primary coolant. A heavy water reactor uses D2O (D = deuterium, 2H) as moderator and primary coolant, and a gas-cooled reactor uses a gas, typically CO2, as primary coolant and graphite as the moderator. Use of either D2O or graphite as the moderator has the advantage that both substances absorb fewer neutrons than H2O. Consequently, the uranium fuel does not need to be enriched (though the reactor can also be run with enriched fuel).

FIGURE 21.20 Fuel spheres used in a high-temperature pebble-bed reactor. The image on the right is an optical microscope image of a fuel particle.

In a high-temperature pebble-bed reactor, the fuel elements are spheres (“pebbles”) roughly the size of an orange (FIGURE 21.20). The spheres are made of graphite, which acts as the moderator, and thousands of tiny fuel particles are embedded in the interior of each sphere. Each fuel particle is a kernel of fissionable material, typically 235U in the form of UO2, surrounded by carbon and a coating of a ceramic material, such as SiC. Hundreds of thousands of these spheres are loosely packed in the reactor core, and helium gas, which acts as the primary coolant, flows up through the packed spheres. The reactor core operates at temperatures considerably higher than those in a light water reactor, approaching 950 °C. A pebble-bed reactor is not subject to steam explosions and does not need to be shut down to refuel. Engineers can remove spent spheres from the bottom of the reactor core and add fresh ones to the top. This design is relatively new and is not yet in commercial use.

Nuclear Waste

The fission products that accumulate as a reactor operates decrease the efficiency of the reactor by capturing neutrons. For this reason, commercial reactors must be stopped periodically to either replace or reprocess the nuclear fuel. When the fuel elements are removed from the reactor, they are initially very radioactive. It was originally intended that they be stored for several months in pools at the reactor site to allow decay of short-lived radioactive nuclei. They were then to be transported in shielded containers to reprocessing plants where the fuel would be separated from the fission products. Reprocessing plants have been plagued with operational difficulties, however, and there is intense opposition in the United States to the transport of nuclear wastes on the nation's roads.

Even if the transportation difficulties could be overcome, the high level of radioactivity of the spent fuel makes reprocessing a hazardous operation. At present in the United States spent fuel elements are kept in storage at reactor sites. Spent fuel is reprocessed, however, in France, Russia, the United Kingdom, India, and Japan.

Storage of spent nuclear fuel poses a major problem because the fission products are extremely radioactive. It is estimated that 20 half-lives are required for their radioactivity to reach levels acceptable for biological exposure. Based on the 28.8-yr half-life of strontium-90, one of the longer-lived and most dangerous of the products, the wastes must be stored for 600 years. Plutonium-239 is one of the by-products present in spent fuel elements. It is formed by absorption of a neutron by uranium-238, followed by two successive beta emissions. (Remember that most of the uranium in the fuel elements is uranium-238.) If the elements are reprocessed, the plutonium-239 is largely recovered because it can be used as a nuclear fuel. However, if the plutonium is not removed, spent elements must be stored for a very long time because plutonium-239 has a half-life of 24,000 yr.

One approach to getting more power out of existing uranium sources and potentially reducing radioactive waste is a fast breeder reactor. This type of reactor is so named because it creates (“breeds”) more fissionable material than it consumes. The reactor operates without a moderator, which means the neutrons used are not slowed down. In order to capture the fast neutrons, the fuel must be highly enriched with both ura-nium-235 and plutonium-239. Water cannot be used as a primary coolant because it would moderate the neutrons, and so a liquid metal, usually sodium, is used. The core is surrounded by a blanket of uranium-238 that captures neutrons that escape the core, producing plutonium-239 in the process. The plutonium can later be separated by reprocessing and used as fuel in a future cycle.

Because fast neutrons are more effective at decaying many radioactive nuclides, the material separated from the uranium and plutonium during reprocessing is less radioactive than waste from other reactors. However, generation of relatively high levels of plutonium coupled with the need for reprocessing is problematic in terms of nuclear nonproliferation. Thus, political factors coupled with increased safety concerns and higher operational costs make fast breeder reactors quite rare.

A considerable amount of research is being devoted to disposal of radioactive wastes. At present, the most attractive possibilities appear to be formation of glass, ceramic, or synthetic rock from the wastes, as a means of immobilizing them. These solid materials would then be placed in containers of high corrosion resistance and durability and buried deep underground. The U.S. Department of Energy (DOE) had designated Yucca Mountain in Nevada as a disposal site, and extensive construction has been done there. However, as of the writing of this book, the DOE has publicly stated that the Yucca Mountain site will not be used for storage, although some members of Congress are fighting this decision. The long-term solution to nuclear waste storage in the United States remains unclear. Whatever the solution finally decided on, there must be assurances that the solids and their containers will not crack from the heat generated by nuclear decay, allowing radioactivity to find its way into underground water supplies.

In spite of all these difficulties, nuclear power is making a modest comeback as an energy source. The threat of global warming has moved some organizations to propose nuclear power as a major energy source in the future. Increasing demand for power in developing Asian countries has sparked a rise in construction of new nuclear power plants in that part of the world (FIGURE 21.21).

GO FIGURE

Which country has the most reactors in operation? Which country has the most reactors under construction? Which country generates the highest percentage of its electricity from nuclear power?

FIGURE 21.21 Number of reactors in operation and under construction for the countries with the largest nuclear power generation capabilities.